According to prior art there are various types of wave energy recovery systems where the arrangements include a base and, for instance, one or more panels pivotally connected to the base to make a reciprocating or oscillating movement about a rotation axis or pivot axis in response to wave forces or tidal forces. The oscillating movement is then converted for example to electric energy with the help of a power-take-off (PTO) unit equipped with a power-take-off (PTO) machinery comprising a generator or alike.
Commonly a PTO unit of prior art wave energy conversion devices is connected more or less permanently to the reciprocating panel of the wave conversion device as is shown for example in the international patent publication No. WO2006/100436 A1. For instance FIG. 6a of the publication shows a device, which has a panel attached by pivot shafts to a base portion, and a driving rod pivotally attached to each side of the panel. Each driving rod connects to a piston inside a hydraulic cylinder, which is pivotally attached to the base portion. In use, the panel oscillates in response to wave action, and the driving rods are driven to cause hydraulic fluid in the hydraulic cylinders to be pressurized by the action of the pistons moved by the driving rods.
Because of massive forces and torques on the pivot shaft, the driving and PTO mechanisms used must be usually hydraulic. However, the reciprocating panel causes still huge reciprocating forces to the driving mechanisms. These reciprocating forces cause extreme fatigue stress to the structure, which leads easily the various fatigue breakdowns. For these reasons also hydraulic driving and PTO arrangements used must be big and robust, and then they are also heavy and expensive.
Another problem with hydraulic driving and PTO arrangements according to prior art is that they require a lot of maintenance and/or repairs, and their lifetime is considerably short. Also the maintenance and/or repairs of the driving and PTO assemblies according to prior art can be time consuming, expensive and even dangerous. Typically, the maintenance and/or repairs must be performed at the production site in-situ, which means that all the work must be done on the bottom of the sea. Thus, for instance, divers are needed to perform the job. The job is slow, difficult and expensive, and can also be dangerous. In addition, since the maintenance mostly must be conducted underwater, the internal components that are usually protected from the water may undesirably get wet during the maintenance or work. Also, the wave energy conversion devices according to prior art must be made inactive during maintenance and repairs. This causes unnecessary shutdown times during which wave energy cannot be recovered.
The maintenance and repairs problem mentioned above has been tried to solve by a solution shown in the international patent publication No. WO2011/026173 A1. This WO publication shows a structure where one or more modular energy transfer mechanisms are connected with a pivot shaft of the wave energy recovery apparatus. The installation and removal of the modular energy transfer mechanism can be done when the panel portion of the apparatus has been lowered into its flat “survival mode” configuration as mentioned in the WO publication. This solution has several disadvantages. Firstly the modular energy transfer mechanisms are connected directly onto the pivot shaft that causes massive forces and torques. Consequently the structure of modular energy transfer mechanism is extremely prone to various fatigue breakdowns and therefore the structure of energy transfer mechanism according to the WO publication must be big and robust, and is therefore also expensive. Another problem is the installation and removal of the modular energy transfer mechanisms. It can be easy and fast only after the panel portion of the apparatus has been lowered into its flat “survival mode” configuration, but that task is extremely difficult, if not almost impossible in prevailing ocean conditions where waves never stop moving.
Also the control of the apparatuses according to prior art has caused problems because the force of waves varies a lot. One prior art solution is shown in the international publication No. WO2014/053839 A1. The arrangement according the WO publication has two hydraulic cylinders with different internal diameters. When the forces of the waves are small the smaller hydraulic cylinder is in use and when the forces of the waves are greater the bigger cylinder can be used. In very powerful conditions both the hydraulic cylinders can be used together. This solution makes the controlling simpler but the pistons move still back and forth in the cylinders driven directly by the reciprocating panel, which cause serious fatigue problems to the structure.
The same problems applies also to the solution shown in the international publication No. WO2013/140042 A2. The publication shows an arrangement where two different sized hydraulic cylinders has been joined together and two different sized pistons make a reciprocating movement in the hydraulic cylinders, one piston in each cylinder. The arrangement is equipped with a control system by which the arrangement can be controlled according to the forces of the waves affecting to the arrangement. Fatigue problems in this arrangement are serious because at least the bigger hydraulic cylinder is considerably big. To avoid the fatigue problems mentioned the structure of the arrangement must be robust and heavy which leads easily to very expensive structures that are also difficult to handle.